Field-effect transistor and manufacturing method thereof

The field-effect transistor (20;20';20") includes a substrate (200) having a first doping region (200a) and an overlying second doping region (200b), wherein the first and second doping regions (200a;200b) have a first conductivity type and wherein the second doping region (200b) has at least one first trench (204) and at least one second trench (212) adjacent thereto. A first epitaxial layer (208;208') is disposed in the first trench (204) and has a second conductivity type. A second epitaxial layer (216;216') is disposed in the second trench (212) and has the first conductivity type, wherein the second epitaxial layer (216;216') has a doping concentration greater than that of the second doping region (200b) and less than that of the first doping region (200a). A gate structure (228;230) is disposed on the second trench (212). A method of fabricating the field-effect transistor (20;20';20") is also disclosed.